Add AgentSessionService and refactor agent routes

Move session-related responsibilities into a new application-layer AgentSessionService (and AgentSessionFeedbackResult dataclass), provide a bootstrap factory (get_agent_session_service), and update agent API routes to call the service instead of accessing ConversationStore directly. Routes now translate ValueError into 404 responses and use service methods for get/list/history/delete/feedback. Also update package exports and docs/READMEs to declare the backend architecture authority, enforce api -> application -> domain ports -> infrastructure boundaries, and call out legacy services/workflows as migration-only. These changes centralize session logic in the application layer and tighten architecture guidance for future backend work.

backend/app/shared/bootstrap.py

@@ -4,7 +4,7 @@ from __future__ import annotations
 
 from functools import lru_cache
 
-from app.application.agent import AgentConversationService
+from app.application.agent import AgentConversationService, AgentSessionService
 from app.application.documents import DocumentCommandService, DocumentQueryService
 from app.application.knowledge import KnowledgeRetrievalService
 from app.config.settings import settings
@@ -162,3 +162,6 @@ def get_perception_service() -> PerceptionService:
         event_store=MockEventStore(),
         retrieval_service=get_retrieval_service(),
     )
+def get_agent_session_service() -> AgentSessionService:
+    """Return agent session service."""
+    return AgentSessionService(conversation_store=get_conversation_store())